1. Field of the Invention
The present invention relates to a power factor correction circuit, and particularly to operation in a critical mode of a bridgeless power factor correction circuit.
2. Description of the Related Art
Conventionally, in order to supply electric power to a load, a power source apparatus in which an AC (alternating current) voltage from an input AC power source is rectified and then converted to a desired AC or DC (direct current) voltage and supplied to the load has been widely used. In this kind of power source apparatus, a power factor correction circuit needs to be provided in order to correct the power factor and reduce the EMI noise generated by the apparatus. Therefore, in a general constitution of a power supply apparatus, a rectification circuit consisting of a diode bridge and a power factor correction circuit consisting of a boost converter circuit are utilized in the input stage.
In recent years, in a power source apparatus, a so-called bridgeless power factor correction circuit, in which a front stage diode bridge is made unnecessary by combining a power factor correction function by a boost operation and a rectification function, has been proposed in, for example, Japanese Patent Application Laid-Open (JP-A) No. 2009-177935. In the power factor correction circuit of JP-A No. 2009-177935, the input stage of the power supply apparatus can be constituted by a simple circuit and the conduction loss of the diode can be reduced, and thus this kind of power factor correction circuit is advantageous over a constitution in which the rectification circuit and the power factor correction circuit are provided separately.
In general, a critical mode is used as an operation mode of a power factor correction circuit. In a critical mode, a point in time at which the reactor current becomes zero is detected during the period in which the main switching element is turned OFF, and the ON/OFF of the main switching element is controlled such that the main switching element is switched ON immediately after the above-mentioned point in time is detected. Therefore, in order to operate the power factor correction circuit in a critical mode, it is necessary to detect the point in time at which the reactor current becomes zero, and as such a current detection technology, a current transformer or the like has conventionally been used.
However, for example, in a current detection technology using a current transformer, there has been a problem in that an additional circuit such as a reset circuit is generally necessary in order to achieve the necessary detection accuracy, and thus the circuit constitution and the control thereof becomes complicated. Another general current detection technology is to connect a current detection resistor to the reactor current path, as in the power factor correction circuit disclosed in JP-A No. 2009-177935. However, in this case, heat generation and power loss in the resistor may become an impediment to miniaturization and efficiency improvement of the power factor correction circuit, and by extension the power supply apparatus itself.